U.S. patent application number 12/359675 was filed with the patent office on 2009-05-28 for method for forming a transparent electroconductive film.
This patent application is currently assigned to ULVAC, INC.. Invention is credited to Satoru Ishibashi, Atsushi Ota, Hirohisa Takahashi, Noriaki Tani, Sadayuki Ukishima.
Application Number | 20090134013 12/359675 |
Document ID | / |
Family ID | 38981548 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090134013 |
Kind Code |
A1 |
Takahashi; Hirohisa ; et
al. |
May 28, 2009 |
METHOD FOR FORMING A TRANSPARENT ELECTROCONDUCTIVE FILM
Abstract
A transparent electroconductive film having a low resistivity is
provided. In a film-forming method of the present invention, a
transparent electroconductive film is formed on a surface of a
substrate by sputtering, in a vacuum atmosphere, a target in which
ZnO is a main component and Al.sub.2O.sub.3 and TiO.sub.2 are added
to ZnO, and then the transparent electroconductive film is annealed
by the heating thereof at a temperature of 250.degree. C. or more
and 400.degree. C. or less. The resistivity of the obtained
transparent electroconductive film is reduced because the film has
ZnO as the main component and Al and Ti added therein. The
transparent electroconductive film formed by the present invention
is suitable as a transparent electrode for the FDP, etc.
Inventors: |
Takahashi; Hirohisa;
(Sammu-shi, JP) ; Ukishima; Sadayuki; (Sammu-shi,
JP) ; Ota; Atsushi; (Sammu-shi, JP) ; Tani;
Noriaki; (Sammu-shi, JP) ; Ishibashi; Satoru;
(Sammu-shi, JP) |
Correspondence
Address: |
KRATZ, QUINTOS & HANSON, LLP
1420 K Street, N.W., Suite 400
WASHINGTON
DC
20005
US
|
Assignee: |
ULVAC, INC.
Chigasaki-shi
JP
|
Family ID: |
38981548 |
Appl. No.: |
12/359675 |
Filed: |
January 26, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP07/64704 |
Jul 26, 2007 |
|
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12359675 |
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Current U.S.
Class: |
204/192.15 |
Current CPC
Class: |
C23C 14/086 20130101;
C23C 14/3414 20130101; C23C 14/5806 20130101 |
Class at
Publication: |
204/192.15 |
International
Class: |
C23C 14/34 20060101
C23C014/34 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2006 |
JP |
2006-205936 |
Claims
1. A transparent electroconductive film-forming method for forming
a transparent electroconductive film on a surface of an object to
be film-formed by sputtering a target having a main component of
ZnO in a vacuum atmosphere, the method comprising; preliminarily
adding a main addition oxide of Al.sub.2O.sub.3 to the target such
that the number of atoms of a main addition element of Al is at
least 1 and at most 10 per 100 atoms of Zn, selecting at least one
kind of secondary addition oxides from a secondary addition oxide
group consisting of TiO.sub.2, HfO.sub.2 and ZrO.sub.2, and
preliminarily adding the selected secondary addition oxide or
oxides to the target such that the total number of atoms of Ti, Hf
or Zr in the selected secondary addition oxide or oxides is at
least 0.5 and at most 5 per 100 atoms of Zn.
2. The transparent electroconductive film-forming method according
to claim 1, wherein after the transparent electroconductive film is
formed, the transparent electroconductive film is annealed by the
heating thereof at a predetermined heating temperature, and the
heating temperature is set at least 250.degree. C. and at most
500.degree. C.
3. The transparent electroconductive film-forming method according
to claim 2, wherein the transparent electroconductive film is
heated in an open air atmosphere in the annealing.
Description
[0001] This is a Continuation of International Application No.
PCT/JP2007/064704 filed Jul. 26, 2007, which claims priority to
Japan Patent Application No. 2006-205936, filed on Jul. 28, 2006.
The entire disclosures of the prior applications are hereby
incorporated herein by reference in their entireties.
BACKGROUND
[0002] The present invention generally relates to a method for
forming a film, and more particularly, to a method for forming a
transparent electroconductive film.
[0003] As transparent electrodes to be employed in FDPs (Flat
Display Panels) such as plasma display panels (PDPS) and liquid
crystal panels, In--Sn--O type transparent electroconductive films
(hereinafter referred to as ITO films) have been conventionally
used. Since the price of indium had recently soared due to the
depletion of indium sources, transparent electroconductive
materials have been sought instead of ITO.
[0004] ZnO based materials have been examined as transparent
electroconductive materials in place of the ITO. However, since ZnO
has a high resistance, it is difficult to use ZnO alone as an
electrode.
[0005] It is known that the resistivity is lowered by adding
Al.sub.2O.sub.3 to ZnO. However, for example, when a film of a
transparent electrode is formed by sputtering a target in which
Al.sub.2O.sub.3 is added to ZnO, the resistivity of the transparent
electrode is several times higher than that of the ITO film, and
reduction in the resistivity is not practically sufficient.
[0006] Although the resistivity is generally lowered by heating
treatment (annealing treatment) after the formation of the
electroconductive film, the resistivity of the ZnO film to which
Al.sub.2O.sub.3 was added was reversely increased by annealing in a
high temperature range in the open air. See patent document No. JP
A 11-236219.
SUMMARY OF THE INVENTION
[0007] The present invention has been accomplished to solve the
above problem, and is aimed at producing a transparent
electroconductive film having a low resistivity by using an
inexpensive and stably suppliable material.
[0008] In order to solve the above problem, the present invention
is directed to a transparent electroconductive film-forming method
for forming a transparent electroconductive film on a surface of an
object to be film-formed by sputtering a target having a main
component of ZnO in a vacuum atmosphere. The method includes:
preliminarily adding a main addition oxide of Al.sub.2O.sub.3 into
the target such that the number of atoms of a main addition element
of Al is 1 or more and 10 or less per 100 atoms of Zn; selecting
one or more kinds of secondary addition oxides from a secondary
addition oxide group consisting of TiO.sub.2, HfO.sub.2 and
ZrO.sub.2; and adding the selected secondary addition oxide or
oxides to the target such that the total number of atoms of Ti, Hf
or Zr in the selected secondary addition oxide or oxides is 0.5 or
more to 5 and less per 100 atoms of Zn.
[0009] The present invention is directed to the transparent
electroconductive film-forming method, wherein after the
transparent electroconductive film is formed, the transparent
electroconductive film is annealed by heating it at a predetermined
heating temperature, and the heating temperature is set at
250.degree. C. or more and less than 500.degree. C.
[0010] The present invention is directed to the transparent
electroconductive film-forming method, wherein the transparent
electroconductive film is heated in an open air atmosphere in the
annealing.
[0011] The main component in the present invention means that the
material as the main component is contained at 50 atom % or more of
the total.
[0012] The present invention is provided as mentioned above, and
since the Al.sub.2O.sub.3 (main addition oxide) and TiO.sub.2
(secondary addition oxide) are added to the target, the transparent
electroconductive film formed by the present invention has ZnO as
the main component, and Al (main addition element) and Ti
(secondary addition element) are added thereto.
[0013] When the secondary addition oxide added to the target is
HfO.sub.2, Hf is added to the transparent electroconductive film as
the secondary addition element. When the secondary addition oxide
is ZrO.sub.2, Zr is added to the transparent electroconductive film
as the secondary addition element. The secondary addition elements
are so-called IVA (4A) group elements.
[0014] The resistivity of the ZnO film is lowered by the addition
of Al, and distortion of ZnO crystals due to the addition of Al is
mitigated by the addition of Ti. Therefore, the dopants (the total
amount of Al and Ti) can be added at high concentrations. As a
result, the resistivity of the transparent electroconductive film
is lowered, as compared to a case in which no Al is added or in
which only Al is added without the addition of Ti.
[0015] Further, an effect similar to that in the case of the
addition of Ti alone is obtained when either one or both of Hf and
Zr as the secondary addition elements are added in place of Ti or
when either one or both of Hf and Zr are added together with
Ti.
[0016] When Al is added alone to a film of ZnO as a donor (electron
donor) at a high concentration, the electron mobility in crystals
decreases, and Al, which is incorporated into the film as it is in
an oxide state, increases. Consequently, the resistivity rises.
According to the present invention, the reduction in the electron
mobility is prevented by adding a different donor or different
donors (such as, Ti) in addition to Al, so that the dopants can be
added at high concentrations.
[0017] When the ZnO film to which Al and Ti are added is heated
(annealed) after the film is formed by sputtering, the film is
activated and the electric resistance decreases. Al is activated
when Al is incorporated into the crystals in the ZnO film in the
form of not an oxide but atoms. However, Al is inactivated by
oxidation when the transparent electroconductive film is heated at
a high temperature of 400.degree. C. or more in the open air
atmosphere.
[0018] Ti is activated at a higher temperature than Al, and is not
oxidized even at a high temperature (for example, 450.degree. C.)
in the open air atmosphere. Therefore, even when the transparent
electroconductive film of the present application is heated at a
high temperature, the resistivity does not rise. Al is not oxidized
in a vacuum.
[0019] Hf and Zr are activated at a higher temperature than Al, and
not oxidized at high temperatures in the open air atmosphere.
Therefore, a similar effect is obtained when either one or both of
Hf and Zr are added as the secondary addition elements in place of
Ti or when either one or both of Hf and Zr are added together with
Ti.
[0020] It is presumed that when the target to which Al.sub.2O.sub.3
and TiO.sub.2 are added is used in such a manner that the ratio of
the number of the atoms of Al to that of Zn is 1% or more and 10%
or less, and the ratio of the number of atoms of Ti to that of Zn
is 0.5% or more and 5% or less, a transparent electroconductive
film having high transparency and a low resistivity may be
obtained.
[0021] The present invention can provide the transparent
electroconductive film having a low resistivity by using the
inexpensive and stably suppliable materials (such as, ZnO,
Al.sub.2O.sub.3 and TiO.sub.2) without the use of indium. Since the
annealing treatment need not be performed in a vacuum atmosphere,
the structure of a film-forming apparatus is simple, and the
processing time in a vacuum chamber is shortened. It is presumed
that when the film is obtained by heating, a similar or higher
quality of the film is obtained. After the film is formed at such a
temperature as causing small damage to a substrate, the resistance
is lowered by the annealing treatment. Such a low temperature
film-forming apparatus is simpler in structure than a high
temperature film-forming apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a sectional view illustrating one example of a
film-forming apparatus according to the present invention.
[0023] FIGS. 2(a) and (b) are sectional views illustrating
film-forming steps of the transparent electroconductive film
according to the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0024] First, an example of steps for producing a target to be used
in the present invention will be explained.
[0025] Three kinds of powdery oxides of ZnO, Al.sub.2O.sub.3 and
TiO.sub.2 are weighed; a mixed powder is prepared, in which ZnO is
a main component and Al atoms and Ti atoms are contained at
predetermined ratios relative to the number of atoms of Zn; and the
mixed powder is preliminarily baked in a vacuum.
[0026] A mixture is prepared by adding and mixing water and a
dispersant into the obtained baked material; and after the mixture
is dried, it is preliminarily baked again in a vacuum. Then, after
the baked material is ground and homogenized, it is molded in a
plate-like form in a vacuum atmosphere, and a plate-like target is
prepared by baking the molded body in the vacuum atmosphere.
[0027] This target has ZnO as the main component, to which
Al.sub.2O.sub.3 and TiO.sub.2 are added; and the ratios of the
numbers of atoms of Zn, Al and Ti contained in the target are the
same as in the above mixed powder.
[0028] Next, steps of forming a transparent electroconductive film
by using the above target will be explained.
[0029] In FIG. 1, a reference numeral 1 generally indicates a
film-forming apparatus to be used in the present invention. This
film-forming apparatus 1 includes a vacuum chamber 2.
[0030] A vacuum evacuation system 9 and a sputtering gas feeding
system 8 are connected to the vacuum chamber 2; and after the
inside of the vacuum chamber 2 is evacuated to a vacuum by the
vacuum evacuation system 9, a sputtering gas is fed into the vacuum
chamber 2 from the sputtering gas feeding system 8, while the
vacuum evacuation is being continued, thereby forming a
film-forming atmosphere at a predetermined pressure.
[0031] The above-mentioned target 11 and a substrate holder 7 are
arranged inside the vacuum chamber 2; and the substrate 21 as an
object to be film-formed thereon is held by the substrate holder 7
in a state such that a surface thereof is directed so as to be
opposed to the target 11.
[0032] The target 11 is connected to an electric power source 5
located outside the vacuum chamber 2. When a voltage is applied to
the target 11 in a state such that the vacuum chamber 2 is set at a
ground potential while the above film-forming atmosphere is being
maintained, the target 11 is sputtered, and sputtered particles are
discharged. A transparent electroconductive film 23 is grown on the
surface of the substrate 21 such that in the transparent
electroconductive film 23, ZnO is a main component and the ratios
of the number of atoms of Zn, that of Al and that of Ti are the
same as in the target 11 (See FIG. 2(a)).
[0033] The film formation is stopped at a time when the transparent
electroconductive film 23 grows to a predetermined film thickness;
and the substrate 21 is taken out from the film-forming apparatus 1
to the open air atmosphere.
[0034] The substrate 21 on which the transparent electroconductive
film 23 is formed is carried into a heater (not shown); and the
transparent electroconductive film 23 is annealed by heating at a
predetermined annealing temperature in the open air atmosphere. In
FIG. 2(b), a reference numeral 24 indicates the transparent
electroconductive film which has performed the annealing treatment.
Since the annealed transparent electroconductive film 24 has a low
resistivity, it can be used as a transparent electrode for an FDP
when that transparent electroconductive film 24 is patterned in a
predetermined shape.
[0035] Unlike the ITO, the transparent electroconductive film of
the present invention can be patterned even after the annealing
treatment.
EXAMPLE
[0036] After a target 11 was prepared under the following
"Preparation condition", a transparent electroconductive film 24 in
Example 1 was formed on a surface of a substrate under the
following "Film-forming condition" by using the target 11.
<Preparation Condition>
[0037] Composition of a mixed powder: the number of atoms of Al=3
and that of Ti=1.5 (per 100 atoms of Zn)
[0038] Preliminary baking (first and second times): 750.degree. C.
in a vacuum atmosphere for 12 hours
[0039] Preparation of a mixture: mixed in a ball mill for 24 hours
by using zirconia balls 10.phi. (particle diameter of 10 mm)
[0040] Drying of the mixture: dried in an oven for 48 hours
[0041] Grinding: manually ground so as to become not more than 750
.mu.m in particle diameter by using a mortar
[0042] Molding and baking of the target: molded and baked at
1000.degree. C. for 150 minutes in a vacuum by hot press
[0043] Size of the target: 4 inches in diameter
<Film-Forming Condition>
[0044] Temperature of a substrate: 160.degree. C.
[0045] Film thickness: 200 nm (2000 .ANG.)
[0046] Sputtering gas: Ar
[0047] Flow rate of Ar: 200 sccm
[0048] Pressure of a film-forming atmosphere: 0.4 Pa
[0049] Voltage applied to the target: 0.8 kW (DC power source)
[0050] Annealing temperature: 200.degree. C. or more and
400.degree. C. or less (in the open air atmosphere)
<Resistivity Measurement>
[0051] As to the transparent electroconductive film 24 in Example 1
after the annealing treatment, the resistivity was measured by a
4-probe low resistivity meter.
[0052] A transparent electroconductive film in the Comparative
Example was prepared under the same condition as in the above
Example 1 except that a target was used, in which ZnO was a main
component and 2 wt % of Al.sub.2O.sub.3 was added (no Ti
contained); and the resistivity of the transparent
electroconductive film was also measured under the same condition
as in the Example 1. The following Table 1 shows the measurement
results thereof together with the annealing temperatures.
TABLE-US-00001 TABLE 1 Measurement of resistivity Resistivity
[.mu..OMEGA. cm] After annealing in open air for 1 hour Before
200.degree. 250.degree. 300.degree. 350.degree. 400.degree. Target
annealing C. C. C. C. C. Example 1 2179 809 543 469 476 608
Comparative 1085 686 645 672 675 590000 Example
[0053] As the transparent electrode for the FDP, the resistivity is
preferably around 500 .mu..OMEGA.cm or less. The measurement
results are as shown in Table 1 such that since the resistivity is
around 500 .mu..OMEGA.cm when the annealing temperature is
250.degree. C. or more and 400.degree. C. or less, the annealing
temperature is preferably 250.degree. C. or more and 400.degree. C.
or less. Further, the above results show that the film obtained in
the Example 1 is transparent and is optically and electrically
suitable as a transparent electrode.
[0054] On the other hand, the resistivity in the Comparative
Example largely exceeded 600 .mu..OMEGA.cm even when the annealing
temperature was varied; and particularly with respect to the film
annealed at the annealing temperature of 400.degree. C. or more,
the oxidation of the transparent electroconductive film proceeded,
and the degradation of the resistance was distinguished. To the
contrary, the resistivity of the transparent electroconductive film
24 in the Example 1 did not significantly increase even when the
annealing temperature was 400.degree. C.
[0055] The above results show that if the transparent
electroconductive film formed by sputtering the target, in which
ZnO is a main component and Al.sub.2O.sub.3 and TiO.sub.2 are added
to ZnO, is annealed at a temperature of 250.degree. C. or more and
400.degree. C. or less, the film suitable for the transparent
electrode can be obtained.
[0056] The above explanation has been made for the case where Ar
gas is used as the sputtering gas, but the invention is not limited
thereto. As the sputtering gas, a Xe gas, a Ne gas or the like can
also be used.
[0057] The method for producing the target 11 is not particularly
limited, and the target 11 to be used in the present application
can be produced by a variety of producing methods that are
ordinarily employed.
[0058] When the annealing treatment is performed in the vacuum
atmosphere, the resistivity becomes lower than when it is performed
in the open air atmosphere. However, since a vacuum chamber to be
exclusively used for the annealing treatment needs to be prepared
for annealing in the vacuum atmosphere, the film-forming apparatus
becomes complicated and expensive. Moreover, as the processing time
inside the vacuum chamber becomes longer due to the annealing time,
the time required for the formation of a film on a single substrate
is longer than in the case where the annealing is performed in the
open air atmosphere.
[0059] As described above, according to the present invention, even
when the annealing treatment is done in the open air atmosphere,
the resistivity becomes practically sufficiently lower for the
transparent electrode, so that the annealing treatment is
preferably performed in the open air atmosphere.
[0060] The transparent electroconductive films 24 formed by the
present invention can be used as transparent electrodes for various
display devices such as FED (Field Emission Display) or the like
besides the transparent electrodes for the PDP and the liquid
crystal panels. Since no problem occurs in the producing process in
the cases of the FED and the PDP even if the annealing temperature
is set at a high temperature of 250.degree. C. or more, the
invention of this application is particularly suitable for the
production of the transparent electrodes in these display
apparatuses.
[0061] If the optimum ranges of an addition amount of
Al.sub.2O.sub.3 (the ratio of the number of atoms of Al with
respect to that of Zn) and an addition amount of TiO.sub.2 (the
ratio of the number of atoms of Ti with respect to that of Zn) to
be respectively added to the target are found, it is estimated that
even if the annealing temperature is less than 250.degree. C., a
low resistivity can be attained.
[0062] In the above, the case where TiO.sub.2 is added to the
target as the secondary addition oxide has been explained, but the
present invention is not limited thereto.
Examples 2 to 6
[0063] Targets 11 in the Examples 2 to 6 were prepared under the
same condition as in the above Example 1 except that addition
amounts of Al.sub.2O.sub.3 and the secondary addition oxides
(TiO.sub.2, HfO.sub.2 or ZrO.sub.2) were changed. After a
transparent electroconductive film 23 was formed under the same
condition as in the above Example 1 by using each target 11, an
annealed transparent electroconductive film 24 was obtained by
heating in a temperature range of 200.degree. C. to 500.degree. C.
in the open air atmosphere.
[0064] Resistivities of the annealed transparent electroconductive
films 24 and the transparent electroconductive films 23 before the
annealing were measured by the method described in the above
"Resistivity measurement".
[0065] The targets 11 of the Examples 2 to 6 are composed of ZnO,
Al.sub.2O.sub.3, TiO.sub.2, HfO.sub.2, ZrO.sub.2, and the following
Table 2 shows the relationships among the numbers of the respective
components per 100 of the components composing the target 11
(figures in a column of ratios of components of target), heating
temperatures and resistance values.
TABLE-US-00002 TABLE 2 Ratios of components of target, heating
temperature and resistivity Ratios of Resistivity (.mu..OMEGA. cm)
components of target Before Heating in open air for 1 hour Zn0
Al.sub.20.sub.3 Ti0.sub.2 Hf0.sub.2 Zr0.sub.2 annealing 200.degree.
C. 250.degree. C. 300.degree. C. 350.degree. C. 400.degree. C.
450.degree. C. 500.degree. C. Example 2 97.0 1.5 1.5 0 0 986 690
503 395 373 381 857 O.L. Example 3 94.0 3.0 3.0 0 0 889 711 529 411
398 402 788 O.L. Example 4 91.0 4.5 4.5 0 0 937 734 548 389 391 371
751 O.L. Example 5 95.5 1.5 0 3.0 0 1168 916 677 504 492 520 1249
O.L. Example 6 95.5 1.5 0 0 3.0 1159 814 625 487 478 475 987
O.L.
[0066] "O.L" in the above Table 2 denotes "over the range", which
shows that the resistivity is so high that it cannot be measured by
the above-mentioned low resistivity meter.
[0067] The above Table 2 shows that when the targets 11 in the
Examples 2 to 6 are used, the results of the Table 2 exhibit "over
the range" at the heating temperature of 500.degree. C.; and thus,
the low resistivities can be obtained at 200.degree. C. or more and
less than 500.degree. C. When the transparent electroconductive
films formed by using the target in the above Comparative Example
were heated at 450.degree. C. or 500.degree. C., the resistivities
were over the range.
[0068] The numbers of atoms of Al, Hf, Ti and Zr contained in the
respective components per 100 atoms of Zn in the target 11 were
determined from the ratios of the components of the targets shown
in the above Table 2, and they were taken as the contents of the
elements. The contents of the elements in the Examples 2 to 6 are
as given in the following Table 3.
TABLE-US-00003 TABLE 3 Contents of elements Contents of elements Zn
Al Ti Hf Zr Example 2 100 3.09 1.55 0 0 Example 3 100 6.38 3.19 0 0
Example 4 100 9.89 4.95 0 0 Example 5 100 3.14 0 3.14 0 Example 6
100 3.14 0 0 3.14
[0069] From the above Table 3 and the above Example 1, in the
Examples 1 to 67 the number of the atoms of the main addition
element (A1) is in a range of 3.09 or more and 9.89 or less
relative to 100 atoms of Zn; and the number of the atoms of the
secondary addition element (Ti, Hf, Zr) is in a range of 1.5 or
more and 4.95 or less relative to 100 atoms of Zn.
[0070] Therefore, it is understood that if the number of the atoms
of the main addition element is 1 or more and 10 or less relative
to 100 atoms of Zn and the number of the atoms of the secondary
addition element is 0.5 or more and 5 or less relative to 100 atoms
of Zn, the transparent electroconductive film 24 optically and
electrically suitable for the transparent electrode can be
formed.
[0071] Although the case has been explained above in which only one
kind of the secondary addition oxides was added to the target 11,
the invention is not limited thereto. Two or more kinds of the
secondary addition oxides from the secondary addition oxide group
consisting of TiO.sub.2, HfO.sub.2 and ZrO.sub.2 may be added to
the same target 11. In this case, the total number of the atoms of
the secondary addition elements (Ti, Hf and Zr) of the secondary
addition oxides added to the target 11 is set at 0.5 or more and 5
or less relative to 100 atoms of Zn.
[0072] Heating of the transparent electroconductive film 23 is not
limited to the heating in the open air atmosphere; and the
transparent electroconductive film 23 may be heated during the
film-formation in the vacuum atmosphere, or in a case subsequent to
the formation of the transparent electroconductive film 23, it may
be heated in the vacuum atmosphere.
[0073] Main causes for the resistance degradation are that the
ionized carrier is oxidized, the oxygen-lacking state cannot be
maintained due to the oxidation, and the carrier does not function
as an n-type semiconductor. Therefore, it is clear that for the
purpose of reducing the resistance, the high-temperature heating in
the open air atmosphere is the severest condition, as compared to
the case of the heating during the film formation and the case of
the heating in the vacuum atmosphere.
[0074] No resistance degradation occurs, even when the annealing
temperature in the vacuum atmosphere is set higher than the
annealing temperature in the open air atmosphere (for example,
500.degree. C. or more). When the heating is performed during the
film formation, the quality of the film can be obtained, which is
equivalent to or higher than the annealing in the open air
atmosphere.
* * * * *